Backport PR matplotlib#28436: Fix is_color_like for 2-tuple of strings and fix to_rgba for (nth_color, alpha) (matplotlib#28438)

Co-authored-by: hannah <[email protected]>

Author: meeseeksmachine

lib/matplotlib/colors.py

@@ -225,7 +225,7 @@ def is_color_like(c):
         return True
     try:
         to_rgba(c)
-    except ValueError:
+    except (TypeError, ValueError):
         return False
     else:
         return True
@@ -296,6 +296,11 @@ def to_rgba(c, alpha=None):
         Tuple of floats ``(r, g, b, a)``, where each channel (red, green, blue,
         alpha) can assume values between 0 and 1.
     """
+    if isinstance(c, tuple) and len(c) == 2:
+        if alpha is None:
+            c, alpha = c
+        else:
+            c = c[0]
     # Special-case nth color syntax because it should not be cached.
     if _is_nth_color(c):
         prop_cycler = mpl.rcParams['axes.prop_cycle']
@@ -325,11 +330,6 @@ def _to_rgba_no_colorcycle(c, alpha=None):
     *alpha* is ignored for the color value ``"none"`` (case-insensitive),
     which always maps to ``(0, 0, 0, 0)``.
     """
-    if isinstance(c, tuple) and len(c) == 2:
-        if alpha is None:
-            c, alpha = c
-        else:
-            c = c[0]
     if alpha is not None and not 0 <= alpha <= 1:
         raise ValueError("'alpha' must be between 0 and 1, inclusive")
     orig_c = c

lib/matplotlib/tests/test_colors.py

@@ -19,7 +19,7 @@
 import matplotlib.scale as mscale
 from matplotlib.rcsetup import cycler
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
-from matplotlib.colors import to_rgba_array
+from matplotlib.colors import is_color_like, to_rgba_array
 
 
 @pytest.mark.parametrize('N, result', [
@@ -1697,3 +1697,16 @@ def test_to_rgba_array_none_color_with_alpha_param():
     assert_array_equal(
         to_rgba_array(c, alpha), [[0., 0., 1., 1.], [0., 0., 0., 0.]]
     )
+
+
+@pytest.mark.parametrize('input, expected',
+                         [('red', True),
+                          (('red', 0.5), True),
+                          (('red', 2), False),
+                          (['red', 0.5], False),
+                          (('red', 'blue'), False),
+                          (['red', 'blue'], False),
+                          ('C3', True),
+                          (('C3', 0.5), True)])
+def test_is_color_like(input, expected):
+    assert is_color_like(input) is expected